In recently increasing interest by society on effective use and reuse of resources, especially polymers produced from plant raw materials are attracting attention. In particular, polylactic acid, a product from a plant-derived raw material, is known recently to have superior properties.
Lactic acid, the raw material for polylactic acid, has been produce by cultivation of a microorganism, generally called lactic bacterium. Typical examples of the lactic bacteria include Lactobacillus and Lactococcus species. Because these lactic bacteria generally show an excellent yield to sugar, but are less resistant to acid, for accumulation of an acidic substance lactic acid in a great amount, the cultivation should be carried out, while the cultivation solution is neutralized for example with an alkali such as calcium carbonate, ammonium hydroxide or sodium hydroxide.
However, such a process gives a lactate salt such as sodium lactate or calcium lactate by neutralization processing with an alkali, requiring processing to convert the lactate salt back to lactic acid in the later purification step and thus additional cost.
Thus, for reduction of the neutralization cost, lactic acid production by acid-resistant yeast was proposed (see Japanese Patent Application Laid Open (JP-A) No. 2001-204464, JP-A No. 2001-204468, Japanese Patent Application National Publication (Laid-Open) No. 2001-516584, JP-A No. 2003-93060 and JP-A No. 2003-259878 and Danilo Porro et al., Biotechnol. Prog., 11: p. 294-298 (1955), Danilo Porro et al., Applied and Environmental Microbiology, 65 (9): p. 4211-4211 (1999) and Satoshi Saitoh et al., Applied and Environmental Microbiology, 71 (5): p. 2789-2792 (2005)). Yeasts naturally do not produce lactic acid and, thus, for lactic acid production by yeast, a gene coding a L-lactate dehydrogenase, an enzyme converting pyruvic acid into L-lactic acid, (hereinafter, abbreviated as L-ldh gene) should be introduced into the yeast by a gene recombination technique.
Bovine-derived L-ldh genes have been studied as the L-ldh gene for introduction to yeast (see Japanese Patent Application National Publication (Laid-Open) No. 2001-516584 and JP-A No. 2003-259878 and Danilo Porro et al., Biotechnol. Prog., 11: p. 294-298 (1955), Danilo Porro et al., Applied and Environmental Microbiology, 65(9): p. 4211-4211 (1999) and Satoshi Saitoh et al., Applied and Environmental Microbiology, 71(5): p. 2789-2792 (2005)), and are reported to be more favorable than lactic bacterium-derived L-ldh genes. The bovine-derived L-ldh genes have a lower yield to sugar of L-lactic acid and, thus, there was a need for further improvement of yield to sugar (see Satoshi Saitoh et al., Applied and Environmental Microbiology, 71(5): p. 2789-2792 (2005)). In addition, improvement of L-lactic acid productivity by mutation of the yeast-derived gene has also been studied. However, mutation of yeast-derived gene often resulted in disadvantages such as elongation of fermentation period and deterioration in sugar-consumption rate (see Japanese Patent Application National Publication (Laid-Open) No. 2001-516584 and JP-A No. 2006-006271).
As described above, L-lactic acid production by yeast is a useful method. However, there exists a need for further improvement in its productivity.